The present invention relates generally to a high-frequency radio signal receiving apparatus for digital communication such as for satellite communication or the like. More specifically, the invention is concerned with a high-frequency radio signal receiving apparatus (hereinafter also referred to as the high-frequency radio signal receiver) which is essentially immune to the influence of noise such as impulsive noise.
In order to have a better understanding of the invention, a known or conventional high-frequency radio signal receiver for digital communication first will be described by reference to FIG. 7 of the accompanying drawing, which shows in a block diagram a general arrangement of the same. In FIG. 7, reference numeral 1 denotes a receiving antenna, 2 denotes a wide-band bandpass filter (hereinafter also referred to as the wide-band BPF), 3 denotes a frequency converter, 4 denotes a narrow-band bandpass filter (hereinafter also referred to as the narrow-band BPF), and numeral 5 denotes a demodulator.
With the structure mentioned above, the high-frequency radio signal receiver operates in the manner described below.
Referring to FIG. 7, upon reception of radio wave signal by the receiving antenna 1, the received signal is fed to the frequency converter 3 by way of the wide-band BPF 2 to be thereby converted into a signal of an appropriate intermediate frequency band. The signal outputted from the frequency converter 3 is caused to pass through the narrow-band BPF 4, whereby only the desired signal is extracted to be subsequently demodulated by the demodulator 5.
Unwanted signal components of the frequencies which are outside of the frequency band for reception are eliminated through cooperation of the wide-band BPF 2 and the narrow-band BPF 4. Among others, in the case of the high-frequency radio signal receivers for mobile facilities, the narrow-band BPF 4 is indispensably required in order to ensure effective utilization of the frequency bands.
In the conventional high-frequency radio signal receiver, there arises a problem mentioned below when disturbing waves such as impulsive noise mixed in the signal of the frequency band for reception are received by the receiving antenna 1. Parenthetically, the term "impulsive noise" as used herein is contemplated to mean such noise which has an extremely short time duration on the order of 10 to 100 .mu.sec and high power or energy level. As the sources for generation of such noise, there may be mentioned transportation or transit facilities, industrial electric/electronic equipment, electric power transmission systems, motor vehicles and others. Incidentally, the impulsive noise is also known as city noise.
Here, it should be noted that such impulsive noise as mentioned above can not be eliminated with the aid of the wide-band BPF 2 and the narrow-band BPF 4 and that the temporal duration of the impulsive noise undergoes elongation during the course of processing performed by the narrow-band BPF 4, thus involving loss of the intrinsic or desired signal components.
Such being the circumstances, the conventional high-frequency radio signal receiver suffers a serious problem that a reception error ratio increases due to intervention of the impulsive noise, which may ultimately lead to the incapability of communication or loss of the communication capability of receiver as a whole.
For coping with the problem mentioned above, a logic circuit designed for bit error correction has heretofore been employed in association with the demodulator 5. However, this logic circuit is very complicated in structure and implemented in a large size. For this reason, a great inconvenience is encountered in installing the logic circuit in the portable-type apparatuses and/or mobiles. Besides, such a logic circuit provides significant increase in the manufacturing cost for the high-frequency radio signal receiver for the digital communication.